In a known manner, such a regulator assembly of this type comprises:                a casing forming a support,        a mode selection knob mounted on the support so as to be movable between at least a first position and a second position,        a regulator intended to be supplied by a source of breathing gas and adapted to supply breathing gas to a breathing cavity in at least the following two operating modes:                    when the mode selection knob is in said first position, the regulator supplies gas to the breathing cavity as long as the pressure in the breathing cavity is not greater than a first pressure relative to ambient pressure,            when the mode selection knob is in said second position, the regulator supplies gas to the breathing cavity as long the pressure in the breathing cavity is not greater than a second pressure relative to ambient pressure, the first pressure being greater than the second pressure.                        
The different positions of the mode selection knob correspond to different operating modes of the regulator that are adopted according to circumstances. The user, typically a pilot, co-pilot, or flight engineer, must manually select the appropriate mode for the circumstances.
When the mode selection knob is in the first position, the regulator supplies the user, via the breathing cavity, with undiluted breathing gas (typically almost pure oxygen) that is pressurized relative to the ambient air of the cockpit. This position is the most suitable for critical situations. It is usually referred to as the “emergency” position and protects the user of the breathing mask from the harmful effects of toxic fumes due for example to a fire in the cockpit.
There are generally other operating modes of the regulator, to reduce the user fatigue caused by the fact that the breathing gas is supplied at a relatively high pressure compared with ambient air, and/or to reduce breathing gas consumption.
Although the known regulator assemblies are satisfactory and provide a good level of safety, the invention aims to further improve the safety they provide.